lecture 2 - The Hardy-Weinberg Theorem A Null model The...

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The Hardy-Weinberg Theorem A Null model The Hardy-Weinberg theorem Describes a population that is not evolving States that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that no other forces are at work.
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What would the genetic composition of a population be, if that population were NOT evolving?
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Definitions - review gene - the functional unit of heredity Allele – alternative forms of a gene locus – location on a chromosome (pl. loci)
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Definitions - review homozygote - same 2 alleles at a locus (AA, A 1 A 1 , etc.) Heterozygote - different alleles at a locus (Aa, A 1 A 2 , etc.) gametes (egg & sperm) are haploid zygote - fertilized egg, diploid
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If p and q represent the relative frequencies of the only two possible alleles (A & B) in a population at a particular locus, then ________________________ where p 2 and q 2 are the frequencies of the alternative homozygous genotypes and 2 pq represents the frequency of heterozygotes Hardy-Weinberg Equilibrium: Expected genotype frequencies
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1. Chance of two independent events happening: Chance of getting heads flipping a coin= 50% Chance of getting heads flipping a coin a second time= 50% Chance two coin flips will both be heads = 25% 50% * 50% = 25% Combined chance of 2 independent events both occurring - multiplication problem
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(general case) (specific case) If we know allele frequencies, can calculate genotype frequencies A 1 A 1 homozygote = p^2 A 2 A 2 homozygote = q 2
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(general case) (specific case)
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2. Chance of 1 or another event happening: Chance of getting 6 on one roll of die = 1/6 Chance of getting a 5 on one roll of a die = 1/6 Chance of getting a 6 or a 5 on one roll is 1/6 + 1/6 = 2/6 = 1/3 Chance of one OR another independent event - addition problem
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(general case) (specific case) If we know allele frequencies, can calculate genotype frequencies heterozygote = 2pq
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(general case) (specific case) A 1 A 1 + A 1 A 2 + A 2 A 2 p 2 + 2pq + q 2 = 1 (general case) 0.49 + 0.42 + 0.09 = 1 (specific case) If we know allele frequencies, can calculate genotype frequencies
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If we know genotype frequencies, can calculate allele frequencies
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A fruit fly population has a gene with 2 alleles, A1 and A2. Tests show that 80% of the gametes produced in the population contain the A1 allele. If the population is in Hardy Weinberg equilibrium, what proportion of the flies carry both A1 and A2? a. 0.04 b. 0.16 c. 0.20 d. 0.32 e. 0.84
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Population in H-W equilibrium is like deck cards Cards be shuffled - proportions of jacks, queens, etc. remain constant Alleles may be shuffled - proportions remain same
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…unless something changes them
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Hardy -Weinberg Equilibrium Principle Null model - model of no effect Based on assumptions: 1. No (natural) selection 2. Mating is random (panmictic) (likelyhood of mating with one mate over the other is equal) 3. Population is infinitely large 4. No gene flow from outside population 5. NO mutation (no new allele)
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Hardy -Weinberg Equilibrium Principle Those assumptions rarely, if ever, met in nature So - why is a null model useful?
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This note was uploaded on 09/13/2010 for the course BILD BILD 3 taught by Professor Woodruff during the Summer '08 term at UCSD.

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lecture 2 - The Hardy-Weinberg Theorem A Null model The...

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